In the backdrop of the widespread public concern about vast sections of Population, e.g., in West-Bengal (India) and Bangladesh, being potentially exposed to consumption of ground water containing high concentrations of arsenic, the urgency for delineating an appropriate treatment strategy for arsenic removal is realized to overcome the grave threat of chronic arsenic poisoning. Recent epidemiogical evidence of the toxicity of inorganic arsenic suggests that the current maximum contaminant level (MCL) may not be sufficiently protective for human health; the probable WHO guideline value of 10 μg/L is based on both estimated health risks and the practical quantitation level. The estimated cost of compliance with more stringent levels (in the range 2-20 μg/L) is quite high. The existing treatment technologies, such as coagulation, softening, and adsorption on alumina or activated carbon and reverse osmosis accomplish arsenic removal to meet the current standard of 50 μg/L. However, imposition of low arsenic MCL is likely to require implementation of new treatment practices or significant modification of the existing treatment practices. It needs to be emphasized that only a few de-arsenification technologies in vogue have been successfully demonstrated in the field. To achieve this stringent target, novel materials need to be developed wherein zeolites are emerging as potential materials, which can be suitably functionalised to target specific pollutants of concern. Zeolites can be best defined as hydrated crystalline, alumino-silicates with uniform pore size, reversible hydration, ion-exchanging sorptive, and sieving ability. These characteristics are to be exploited for targeting anions in specific arsenate/arsenite ion. It is really challenging to tackle anionic pollutants using functionalised faujasitic zeolites and has been achieved by modifying the surface of the zeolites to selectively target anion.
Reference may be made to Li, Zhaohui, et al. (2000) wherein they studied the removal of oxyanions viz. nitrate, arsenate, chromate, etc. using surface-modified clay. Natural kaolinite was treated with hexadecyltrimethylammonium bromide (HDTMA-Br) to a level twice than that of the cation exchange capacity (CEC). Sorption of each oxyanions was well described by the Langmuir isotherm. Desorption of bromide counter ion indicated that each of the oxyanions was retained by ion exchange on HDTMABr bilayer formed on the organo-kaolinite. It was unaffected by solution in the pH range of 5-9, but at pH 11 the bilayer was affected due to competition of OH− for the anion exchange sites. The results demonstrated that properly prepared organoclays could remove oxyanions as well as non-polar organics, from contaminated water.
Reference may be made to Krishana et al. (2001) wherein adsorption of chromate by surface-modified clays like kaolinite and montomorillinonite modified with cationic surfactant hexadecyltrimethylammonium bromide was reported. It was observed that the amount of chromate absorbed is dependent on pH and that the removal goes on decreasing with increasing pH and it becomes negligible over pH 8 (23). Clays also have the inherent disadvantage of swelling and shrinkage associated with them and therefore cannot be used effectively for treatment purposes.
Reference may also be made to Li et al. (1998) wherein it has been reported that planar nitrate sorbs more on surfactant-modified zeolite surface than tetrahedral chromate. In the presence of sulphate or nitrate, chromate sorption is hindered due to competition or sorption sites, quantitative sorption of nitrate and chromate and desorption of bromide indicate that the sorption of oxyanions is primarily due to surface anion exchange. However, the material has limited exchange potential due to limited exchange capacity of the natural zeolite.
Reference may be made to Tucker et al. (1995) wherein a method for removing anions from water is provided, wherein a complexing agent such as a cationic polyelectrolyte is added to unreacted water. The cationic polyelectrolyte complexes with anions, such as chromate and are filtered. The complex is then treated with a regeneration agent, such as barium chloride or lead chloride to precipitate ions and to regenerate complexing agent, which can be reused for water treatment. However, the method faces difficulty in its practical application for drinking water wherein the requirement of an ultra filtration membrane to retain the retention complex and then regeneration of the same makes the process tedious and expensive.
Reference may be made to Hamann et al. (1994) wherein coagulation and lime softening was reported extensively for the removal of arsenic. Adsorption-coprecipitation with hydrolysing metals such as Al3+ and Fe3+ is the most commonly used treatment technique for removing arsenic from water. Iron coagulation achieves high As (III) and As (V) removal than alum coagulation. This mode of treatment generates sludge, disposal of which, in turn maybe a problem.
Willey (1987) studied the use of treatment processes such as reverse osmosis (RO), ion exchange, adsorption or electrodialysis. These methods are quite expensive for usage in domestic purposes.
Reference may be made to Huang et al. (1989) wherein arsenic removal by adsorption has also been evaluated extensively. It was reported therein that activated carbon adsorption was not effective for the removal of arsenic, but pre-treatment of activated carbon with iron salts has been shown to improve the sorption capacity of arsenic. This method suffers form drawback of having lower and limited loading of iron, which in turns decreases, the adsorption capacity.
Reference may be made to Bowman et al. (1998) wherein the uptake of the surfactants hexadecyltrimethylammonium bromide (HDTMA-Br) by a natural clinoptilolite rich zeolite and subsequent retention of aqueous solutes was studied. SMZ has shown weak sorption capacity for cations such as Pb(II), Sr(II), and strong sorption for anions such as CrO42-. This is in contrast to the natural zeolite, which are good cation exchangers. The results suggested formation of stable HDTMABr bilayer on the external surface of zeolite, which retained anions via anion exchange mechanism. SMZ thus proved to be a useful sorbent for anion removal. However, natural zeolite by virtue of its low exchange capacity and other impurities associated, sorbs anion to a lower degree, which can be improved by using synthetic zeolites.
Reference may be made to Alkesaddra et al. (2000) wherein adsorption of sulphate, hydrochromate, and dihydrogen phosphate, anions on the surfactant-modified clinoptilolite was studied. The SMZ was prepared by the adsorption of cis-1 aminooctaden-9 (oleyamine) on both modified and unmodified natural clinoptilolite. It was observed that oleyamine adsorbed on H+-clinoptiolite by protonation of NH2 group has shown strong anion adsorbing tendency as compared to Ca and Na-clinoptilolite derivatives, which are weak anion adsorbents. The differences in anion adsorption are attributed to the fact that oleylamine forms hydrogen bonding with Ca and Na-clinoptilolite and thus yields insufficient adsorption sites for anions.
Reference may be made to U.S. Pat. No. 6,326,326, (Dec. 4, 2001) Feng et al. wherein, they developed an organized assembly of functional molecules with specific interfacial functionality (functional group(s)) was attached to available surfaces including within mesopores of a mesoporous material. The method of which the present invention avoids the standard base soak that would digest the walls between the mesopores by boiling the mesoporous material in water for surface preparation then removing all but one or two layers of water molecules on the internal surface of a pore. Suitable functional molecule precursor is then applied to permeate the hydrated pores and the precursor then undergoes condensation to form the functional molecules on the interior surface(s) of the pore(s). These materials are reported to perform at very low concentration and have high adsorption capacity.
Reference may be made to U.S. Pat. No. 5,833,739, (Nov. 10, 1998) wherein, Klatte, et al. developed a process for coating zeolite crystals with paraffin, a wax other than paraffin, a fat or oil, or a mixture of at least one QAC and a wax, fat, or oil was reported. Preferably, the crystals were dehydrated until they have about 5% moisture content, and were then mixed with paraffin to produce paraffin-coated zeolite crystals having a desired content of paraffin. Zeolite crystal having pores coated with wax, fat, oil, or a mixture of at least one QAC and wax, fat, or oil is positively charged and has tendency to attract anions. The coating of paraffin/wax applied to zeolite crystal prior to adsorption of QAC may limit its application for removal of anions; in specific, arsenic from drinking water.
Reference may be made to Zhaohui Li et al. (1999) wherein, Natural zeolite and ZVI were homogenized and pelletized to maintain favourable hydraulic properties while minimizing materials segregation due to bulk density difference. The zeolite/ZVI pellets were modified with the cationic surfactants hexadecyltrimethyl ammonium bromide to increase contaminant sorption and thus the contaminant concentration on the solid surface. Results of chromate sorption/reduction indicate that the chromate sorption capacity of palletised SMZ/ZVI is at least 1 order of magnitude higher than that of zeolite/ZVI pellets.
The SMZ material developed in the present invention overcomes the following drawbacks of the conventional materials in vogue:                Lack of selectivity of conventional adsorbents for arsenic at low concentrations        Lack of versatility of conventional for adsorbents for sorption of wide range of pollutants ranging from cationic to anionic        Limited efficiency of conventional adsorbent        Frequent regeneration and disposal by virtue of its possible conversion to value added ceramic precursors by heat treatment        Transfer of arsenic by stabilization of SMZ at higher temperature        Cost-effectiveness of other adsorbents by offering single unit for wide array of pollutants vis-à-vis multiple units required for targeting wide array of pollutants        Sludge generation associated with conventional chemical method viz. alum treatment, chemical precipitation etc.        Hazardous chemical handling etc. by providing technically non-tedious and clean process.        Improvisation in quality of life vis-à-vis improved quality of water.        